1. Field of the Invention
The present invention relates to a substrate and a method of fabricating the same. More particularly, the present invention relates to a substrate having an improved thermal and electrical conductivity and a method of fabricating the same.
2. Description of the Related Art
The roles played by information products in this highly commercialized society are increasingly important. With the rapid development of information products, most electronic products have multiple functions and an increasingly miniaturized and streamlined body. Furthermore, the quality of the product is also increasingly stable. However, with the miniaturization of products, an efficient method of dissipating heat away from a high-power chip such as a power chip or a light-emitting diode and hence stabilizing its operation has become a dominant issue in the design of electronic products. For a package structure having a chip disposed on a substrate, the substrate constitutes one of the pathways for dissipating heat away from the chip and thus plays a critical role in the performance of the device. In other words, the heat-dissipating capacity of the substrate will directly influence the reliability of chip operation.
FIG. 1 is a schematic cross-sectional view of a conventional package structure having a substrate with good thermal conductivity. As shown in FIG. 1, the substrate 110 includes a patterned circuit layer 112, an insulating layer 114 and a metallic panel 116. The insulating layer 114 is located between the patterned circuit layer 112 and the metallic panel 116. A heat source 120 (a heat-generating device) is connected to a plurality of leads 130 that are bonded to the patterned circuit layer 112 on the substrate 110 through a soldering material 140. Any heat generated by the heat source 120 can be dissipated through the solder material 140, the patterned circuit layer 112, the insulating layer 114 and the metallic panel 116 as indicated by the arrow 150. Alternatively, heat from the heat source 120 can be dissipated through the leads 130, the solder material 140, the patterned circuit layer 112, the insulating layer 114 and the metallic panel 116 as indicated by the arrow 160.
In general, the insulating material 114 is fabricated using a substance including Fr-4, BT, Polyimide and PET, for example. Typically, these materials have a coefficient of thermal conductivity smaller than 1 W/m·° K. Even the improved thermally conductive PP has a coefficient of thermal conductivity smaller than 10 W/m·° K. Since the insulating layer 114 has such a low thermal conductivity and both heat-dissipating pathway 150 and 160 need to go through the insulating layer 114, the heat-dissipating efficiency of the respective heat-dissipating pathways 150 and 160 are not fully optimized. Because the heat-dissipating pathways can hardly provide the heat-dissipating rate demanded by the heat source 120, the performance and life of the heat source 120 will be adversely affected.
In addition, the patterned circuit layer 112 has a small cross-sectional area perpendicular to the current flow direction so that a higher resistivity is encountered. Hence, a high performance electrical transmission in the current flow direction is prevented.